1. Technical Field
The present invention relates to magnetoresistive random access memory (MRAM) and, more particularly, to constricting switching current in the tunnel layer of MRAM devices.
2. Description of the Related Art
Spin torque magnetoresistive random access memory (MRAM) uses a two-terminal device having a pinned layer, a tunnel barrier, and a free layer in a magnetic tunnel junction stack. The magnetization of the pinned layer is fixed in a given direction (e.g., “up”), and current passed through the tunnel layer causes the free layer to assume a desired magnetization. The state of the device is then determined according to whether the free layer has a magnetization that is parallel to, or anti-parallel to, the pinned layer.
The physical mechanism employed for writing is spin torque transfer. While a general current is generally unpolarized with respect to spin, in an MRAM device the pinned layer has the effect of polarizing the electrons' spins. These electrons transfer their angular momentum to the free layer at the junction between the free layer and the tunnel layer, reinforcing or reversing the magnetization of the free layer.
A current is therefore used to set a desired polarity to the device, while a smaller current is used to read the stored polarity. The current is applied across the magnetic layers and the tunnel barrier, where the magnetic layers are conductive, but the tunnel barrier is an insulator. However, the tunnel barrier is made thin enough that electrons can pass through the barrier via quantum tunneling. The read current depends on the relative orientations of the magnetizations of the pinned and free layers, with a resistance of the device generally being higher when the magnetizations are anti-parallel and lower when the magnetizations are parallel. The switching current, being substantially larger than the read current, represents the bulk of the device's power output.